Mode-division multiplexing (MDM) is a technique which has become popular recently as a solution to the slowed growth of bandwidth density for optical communication. As an analogy: in fiber optic cables, wavelength division multiplexing (WDM) technique allows many wavelengths of light to carry signals simultaneously on one fiber optic cable. Recently, to allow even greater bandwidths in one fiber optic cable, significant effort has been put forth to exploit spatial modes by using multimode fibers.
In on-chip photonic integration technology, silicon waveguides have greatly matured in the past decade. Silicon waveguides offer major benefits for optical interconnects, e.g., in datacenters. One of such benefits is to eliminate the need for power inefficient optical-electrical-optical conversion. Both optical fiber and WDM have been implemented on-chip. On-chip MDM is a relatively new effort, but promises enormous increases in on-chip bandwidth by utilizing the spatial modes of integrated waveguides to carry separate optical channels. Recently, multiplexers and demultiplexers for MDM have been demonstrated. Multiplexers take signals on multiple single-mode waveguides and multiplex these signals into a single, multimode waveguide. Demultiplexers perform the reverse process of the Multiplexers.